What are the safety regulations for source PCB?
Switch withstand voltage and leakage requirements
When the input and output voltages of the switching power supply exceed 36V AC and 42V DC, electric shock needs to be considered. Safety regulations stipulate that the leakage between any two accessible parts or any accessible part and one pole of the power supply shall not exceed 0.7mAp or 2mA DC.
When the input voltage is 220V switching power supply, the creepage distance between the hot and cold grounds must not be less than 6mm, and the distance between the two terminal lines must be greater than 3mm.
The withstand voltage between the primary and secondary of the switching transformer requires AC 3000v, and the leakage current is set to 10mA. For a 1-minute test, the leakage current must be less than 10mA.
The withstand voltage of the input terminal of the switching power supply to ground (casing) is AC 1500V, set the leakage current to 10mA, and conduct a 1-minute withstand voltage test. The leakage current must be less than 10mA.
The output terminal of the switching power supply has a withstand voltage of 500V DC to the ground (casing), and the leakage current is set to 10mA. After a 1-minute withstand voltage test, the leakage current must be less than 10mA.
Switch safety creepage distance requirements
The safety distance between the side and secondary sides of the two lines: 6mm, plus 1mm slotting, it will also be 4.5mm.
The safety distance between the side and the secondary side in the third line: 6mm, plus 1mm slotting, it also needs 4.5mm.
The safety distance between the two copper foils of the fuse is >2.5mm. Adding 1mm slot also requires 1.5mm.
The distance between L-N, L-GND, N-GND. is greater than 3.5mm.
The primary filter capacitor pin spacing is >4mm.
Safety distance between primary and secondary levels》6mm.
Switching power supply PCB wiring requirements
Between copper foil and copper foil: 0.5mm
Between copper foil and solder joint: 0.75mm
Between solder joints: 1.0mm
Between copper foil and board edge: 0.25mm
Between hole edge and hole edge: 1.0mm
Between hole edge and plate edge: 1.0mm
Copper foil line width》0.3mm.
Turning angle 45°
Equal spacing is required when routing parallel lines.
Safety requirements for switching power supply
Find out the fuse required by safety regulations from the safety regulations. The creepage distance between its two pads is >3.0mm (min). After a short circuit occurs in the rear stage, the safety capacitors X and Y must be connected. It takes into account the withstand voltage and allowable leakage current. The leakage current of the equipment in the subtropical environment should be 0.1uF. After a normally working device is powered off, the voltage between its plugs shall not exceed 42V within 1 second.
Switching power supply protection requirements
When the total output power of the switching power supply is greater than 15W, a short circuit test should be performed.
When the output terminal is short-circuited, overheating or fire cannot occur in the circuit, or the burning time is within 3.
When the distance between adjacent lines is less than 0.2mm, it can be considered a short circuit.
Electrolytic capacitors should be subjected to short-circuit tests. At this time, because electrolytic capacitors can easily fail, attention should be paid to the components during short-circuit tests to prevent fire.
Two metals with different properties cannot be used as connectors because electrical corrosion will occur.
The contact area between the solder joints and the component pins is larger than the cross-sectional area of the component pins. Otherwise it is considered as virtual soldering.
Devices that affect switching power supplies-electrolytic capacitors
Electrolytic capacitors are unsafe devices in switching power supplies and have an impact on the mean time between failures (MBTF) of switching power supplies.
After an electrolytic capacitor is used for a period of time, the capacitance will decrease and the ripple voltage will increase, so it is easy to heat up and fail.
When high-power electrolytic capacitors heat up and fail, they often cause explosions. Therefore, electrolytic capacitors with a diameter greater than 10mm must have explosion-proof functions. Electrolytic capacitors with explosion-proof function have cross-slots on the top of the capacitor case and vent holes at the bottom of the pins.
The service life of the capacitor is mainly determined by the temperature inside the capacitor, and the temperature rise of the capacitor is mainly related to the ripple current and ripple voltage. Therefore, the ripple current and ripple voltage parameters generally given by electrolytic capacitors are in The ripple current value under specific operating temperature (85°C or 105°C) and specific service life (2000 hours), that is: under the conditions of this ripple current and ripple voltage, the life of the electrolytic capacitor is only 2000 hours. When the service life of the capacitor is required to be greater than 2000 hours, the service life of the capacitor needs to be designed according to the following formula.
The following is the formula for calculating the service life of electrolytic capacitors based on the Arrhenius theory:
In the formula:
L: actual average service life;
L b: basic life span at operating temperature, generally 2000 hours (refer to the information);
T max: Working temperature, usually 85℃ or 105℃;
T a: actual ambient temperature;
T h: The temperature required to halve the life span, usually 10°C or 12°C;
ΔT jo: After adding the rated ripple current, the internal temperature rise of the capacitor is related to the packaging structure of the capacitor. The value range is: 3.5~10℃
ΔT j: The temperature rise inside the capacitor after adding the actual ripple current.
In the formula:
F: Frequency coefficient. The frequency coefficient is listed in the product catalog or specification book. It is generally defined that F=1 when the operating frequency is 120Hz or 100kHz. At other operating frequencies, F is greater than or less than 1;
Io: Rated ripple current at operating temperature;
I: Actual ripple current.
According to the calculation results of the above formula, every time the temperature of the electrolytic capacitor increases by 10°C to 12°C, the life of the electrolytic capacitor will be reduced by half. For example, if the service life of an electrolytic capacitor is required to reach 50,000 hours, the operating temperature of the electrolytic capacitor should not exceed 55°C.
#safety #regulations #source #PCB
- Input-output relationship and circuit application diagram of voltage follower
- How to keep driving heavy loads when the voltage drops?
- Novel theory-based evaluation gives a clearer picture of fusion in the sun
- What are the disadvantages of IC temperature sensors?
- Control transformer overcurrent protection, grounding and applications
- What is the difference between chip packaging and SMD?
- Design and application of dry multi-channel priority amplifier
- Can igbt directly replace thyristor? What will be the impact of IGBT directly replacing thyristor?
- Easily understand the avalanche effect of power MOSFETs
- Google’s AI isn’t too ‘woke.’ It’s too rushed